Just bought one of these from just campers as they have ones made for taller vehicles with 2.16m door height to make access easier

You can always use a dehumidifier. There was a study done here in South Africa about car storage at the car factories. From good to bad: Purpose built indoor garage. Out in the open (sun damage biggest problem) Shaded car ports (here they get moisture but the sun cannot burn it off) This is a big problem in our winter rainfall areas.

I used pry bars as tent pegs on mine, and self drilling screws through the lower bars to stop them getting pulled up. It lasted 3 years before getting shredded in a storm. Condensation I never solved.

I have the same garage but the next size up, there's loads of room to work. I attached all the feet to the ground with hammer in anchors and have survived one winter to date without incident.

Interesting... I don't get condensation on my series... Might be the missing door seals haha

I have the cheapest one i could find on Ebay, it has survived (barely) 10 years, nuts and bolts replaced for stainless, poles bent in windy conditions. I live next to a stream so condensation is horrendous, even with vents at the top of the tent. The cover was no longer waterproof after about 12 months so I purchased some vinyl canvas and laid it across the poles underneath the cover. Hopefully my project will be finished this year so the tent will become a poly tunnel green house!!

If there is any hope of electricity you can consider a dehumidifier with either Option One a plug in timer for "pulse" operation 15 minutes on, 30 minutes off with an air supply duct lashed up into an open window. Probably better is Option Two, the dehumidifier inside the vehicle with an industrial humidistat. The problem with whatever you do is likely to be cold weather icing of the dehumidifier despite any latent heat gains from moisture removal plus the heat energy gains from the compressor and you need a continuous condensate drain. Desiccant chemical type will need to be on a mammoth scale given the leakiness of the door seals. Heating is generally ruinously costly as a means to keep metal above dew point . Spoken here from both dealing with exhibits at a steam museum and attempts over decades trying to prevent mould on car interiors while they deteriorate from classics into projects...

One tiny dehimidifer isnt going to work you should have had about five dotted around the vehicle.

Maybe most of the humidity is coming from the ground through the soil? Have you tried covering the floor with something? Probably a lot of moisture is evaporating from the bottom up and condensing on the Defender. Just a thought....

Can you (1) get a large lightweight hikers fly that can be set up with a couple of posts and ropes over the existing structure to give it a "tropical roof" in Land Rover terms? This should move the condensation out a layer. (2) install a diesel heater in the Defender and run it an hour a day? or as required (3) leave the windows and vents cracked open and run an industrial fan. (4) form a local land Rover club and get a Club House, ie : hire a factory between 10 people to store projects in. (5) can you still get the ex-MOD inflatable garages?

Bracing idea is very good. Perhaps condensation could be avoided with a 'tent fly' type concept, another tarp over the garage or a simple form of insulation on the inside. Maybe a car cover now would suffice. Just thoughts whilst shaving.

Interestingly agricultural sheds use hit'n'miss on the sides. This allows airflow while preventing the worst of the weather. Just a thought. Obvs a heated garage is best...

Could you not rest each 'leg on a concrete 'block', say 12" high, that way you would keep a good airflow, and have extra headroom?

Do the poles not fit through the holes in those bricks? This would add weight to each leg .. just a thought from afar

Could you get a vw t6.1 in there? How wide is your vehicle?

get one of those cheap chinese heaters and blow the hot air in keep landy warm and dry they dont cost much to run once all the big holes are sealed up just a thought i used to keep my motorbike ina heated garage it was imaculate worth a go